Intergalactic Medium

The intergalactic medium is the hot, X-ray emitting gas that permeates the space between galaxies. With temperatures of millions of degrees Kelvin and containing less than one atom per cubic meter (a density less than 10-27 kg/m3), intergalactic space is one of the hottest and most rarefied environments in the Universe.

Originally it was assumed that the intergalactic medium was composed entirely of primordial hydrogen and helium left over from the Big Bang. However, in the 1970s, X-ray observations revealed large quantities of metals mixed in with the hydrogen and helium. These metals could only have been made by stars within the galaxies, and somehow later ejected into intergalactic space.